The 555 is a great part but I've come across a few designs that simply worked better with a micro.

When using a 555 as a timer its repeatability is only as good as the capacitors used and when timing 20 seconds they can get large. I confess that I never did the math to find out just how big the timing cap would be in this situation.

The 555 also has another quirk when the circuit is first powered. The first interval is always considerably longer than the rest. It triggers at 0.33*VCC and 0.67*VCC and when you first turn it on it takes longer because it has to charge the cap to 0.67*VCC from zero. Does it matter for wipers? Probably not.

There are ways around all of these problems, but with micros being so cheap now it wasn't worth the trouble. Only 8 pins and with an internal oscillator! (granted, an RC).